An apparatus as described above is disclosed in U.S. Pat. No. 5,039,397. According to this publication the dipleg of the so-called close-bottomed cyclone separators are submerged in the catalyst bed to provide a seal, preventing cracked vapor to flow through the dipleg.
A problem often encountered is that pressure surges occur in the reactor riser due to for example equipment malfunctions, sudden vaporisation of water present in the hydrocarbon feedstock and/or various unit pressure upsets. In the event of a pressure surge the pressure imposed on the system by the catalyst bed around the submerged end of the dipleg as described in U.S. Pat. No. 5,039,397 will not be sufficient to prevent cracked vapour from flowing through the dipleg. This phenomenon is also referred to as “carry under” of the gasses flowing through the cyclone. This is disadvantageous because the cracked vapor contains relatively higher amounts of coke precursors than for example the hydrocarbons which are normally entrained by the catalyst which is discharged through the dipleg. These coke precursors give rise to an undesirable coke formation in the open volume above the dense fluidized bed of the stripping zone. This coke formation can result in that an FCC reactor needs to be shut down before the end of a process run. Removal of the coke is furthermore very laborious and time consuming.
The above problem is overcome by positioning a horizontal plate just below the lower open end of the dipleg. The plate ensures that in the event of a pressure surge no drastic increase in downflow of cracked vapor occurs via the dipleg. It is believed that this is achieved due to the back-pressure resulting from the catalyst being pressed, by the pressure surge, through the restricted opening between the plate and the catalyst discharge opening of the dipleg. Typically the plate has a diameter of more than 1.5 times the dipleg diameter. Examples of such prior art devices are illustrated in U.S. Pat. No. 2,958,653 and U.S. Pat. No. 5,139,748.
A disadvantage of the reactor vessel according to the prior art is that the plates occupy a large horizontal space in the reactor vessel. This results in that the vessel needs to have a larger diameter or that less diplegs and thus less cyclones can be used in one reactor vessel. Such geometrical limitations are for example encountered when more than one primary cyclone dipleg and more than one secondary cyclone dipleg are submerged in the dense phase fluidized stripping bed.